Conventionally it is known for an automatic speed change of a self-propelled motor vehicle, to have the output side of an engine connected to a torque converter, the output side of the torque converter connected to the input side of an automatic transmission having a plurality of speeds, and the output side of the transmission connected to the right and left driving wheels through a differential mechanism. It is also known that where the vehicle speed is increased or decreased in accordance with a relation between engine torque and the vehicle speed, transmission shift points are set on the basis of the vehicle speed. It is also known for the vehicle speed to be detected using the transmission output shaft rotational speed so that when the vehicle speed has exceeded the shift point, the transmission is automatically shifted to the next speed.
For example, as indicated by a dotted line in FIG. 5, gear-shift points f.sub.1N to f.sub.6N are set in accordance with a relation between engine torque and vehicle speed in the forward first speed F.sub.1 to forward seventh speed F.sub.7 ; when the vehicle speed has exceeded the shift points, the transmission is automatically shifted to the next speed.
However, in a self-propelled motor vehicle equipped with a retarder which is used when continuously running down a slope, cooling oil is supplied to cool the retarder during retarder operation. Therefore, during the retarder operation, the engine speed must be raised to increase the amount of the cooling oil discharged from a cooling oil discharge pump, and to ensure that sufficient oil is supplied to the retarder.
According to this conventional technique, the shift points indicated by the full lines in FIG. 5 is moved to a higher vehicle-speed side (indicated by the dotted lines) so that the engine speed can be increased as compared with normal driving.
According to the above-described method of changing the vehicle speed, when the accelerator is released in an attempt to control the speed at the top of a hill or at the beginning of a rough road, for example when a driver releases his foot from the accelerator pedal, the shift point is moved to the high vehicle-speed side in response to an accelerator off signal, thereby allowing an automatic downshift even when the vehicle speed has dropped only a little.
In more detail, as shown in FIG. 5, when the vehicle is traveling (for example) at the point D at a vehicle speed of V.sub.4D and in gear F.sub.4, and the accelerator is released in an attempt to control the vehicle speed at the beginning of a rough road, the shift mode pattern will be changed from the dotted line to the full line. Therefore, in the ordinary shift mode indicated by the dotted line, the amount of deceleration before shifting down to F.sub.3 is V.sub.4D -V.sub.3N =d.sub.N. However, when the vehicle is running in a high vehicle-speed mode indicated by the full line, the deceleration range becomes as narrow as V.sub.4D -V.sub.3H =d.sub.H (d.sub.H &lt;d.sub.N).
If the transmission is operated as described above, shifting frequency will increase, with the result that the longevity of the transmission will not only be reduced but impart an unpleasant drive feeling to the driver because of the down-shift being effected regardless of the driver's intentions.